Leishmaniae are protozoa belonging to the family Trypanosomatidae. On clinical grounds, they have been divided into three major groups: cutaneous, mucocutaneous and visceral. Visceral leishmaniasis is caused by intracellular protozoa of the Leishmania donovani complex and is transmitted by sandflies.

Parasites (promastigote forms) inoculated by the vector invade macrophages by induced phagocytosis. In a slowly progressive disease, spleen, liver, bone marrow and lymph nodes become infected. The disease is often fatal.